Programming memory devices

ABSTRACT

A target memory cell of a memory device is programmed by applying a programming voltage to a word line that includes the target memory cell, determining whether the target memory cell is programmed, and increasing the programming voltage by a step voltage if it is determined that the target memory cell is not programmed. An initial programming voltage and the step voltage are each selectable after fabrication of the memory device.

RELATED APPLICATION

This is a divisional application of application Ser. No. 11/126,790, titled “PROGRAMMING MEMORY DEVICES,” filed May 11, 2005, now U.S. Pat. No. 7,269,066 which application is assigned to the assignee of the present invention and the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory devices and in particular the present invention relates to programming memory devices.

BACKGROUND OF THE INVENTION

A typical flash memory comprises a memory array that includes a large number of memory cells. Each of the memory cells includes a floating gate embedded in a MOS transistor. The cells are usually grouped into sections called “erase blocks.” Each of the cells within an erase block can be electrically programmed selectively by tunneling charges to the floating gate. The negative charge is typically removed from the floating gate by a block erase operation, wherein all floating gate memory cells in the erase block are erased in a single operation.

Two common types of flash memory array architectures are the “NAND” and “NOR” architectures, so called for the resemblance which the basic memory cell configuration of each architecture has to a basic NAND or NOR gate circuit, respectively. In the NOR array architecture, the floating gate memory cells of the memory array are arranged in a matrix. The gates of each floating gate memory cell of the array matrix are connected by rows to word select lines (word lines) and their drains are connected to column bit lines. The source of each floating gate memory cell is typically connected to a common source line. The NOR architecture floating gate memory array is accessed by a row decoder activating a row of floating gate memory cells by selecting the word line connected to their gates. The row of selected memory cells then place their stored data values on the column bit lines by flowing a differing current if in a programmed state or non-programmed state from the connected source line to the connected column bit lines.

A NAND array architecture also arranges its array of floating gate memory cells in a matrix such that the gates of each floating gate memory cell of the array are connected by rows to word lines. However, each memory cell is not directly connected to a source line and a column bit line. Instead, the memory cells of the array are arranged together in strings, typically of 8, 16, 32, or more each, where the memory cells in the string are connected together in series, source to drain, between a common source line and a column bit line. The NAND architecture floating gate memory array is then accessed by a row decoder activating a row of floating gate memory cells by selecting the word select line connected to their gates. The word lines connected to the gates of the unselected memory cells of each string are driven to operate as pass transistors, allowing them to pass current in a manner that is unrestricted by their stored data values. Current then flows from the source line to the column bit line through each floating gate memory cell of the series connected string, restricted only by the memory cells of each string that are selected to be read. Thereby placing the current encoded stored data values of the row of selected memory cells on the column bit lines.

Two common programming techniques for NAND architecture Flash memories are the “boosted bit line” and the “boosted source line.” In these techniques a high voltage is applied to the gate of a selected floating gate transistor of a string, while the remaining transistors are turned on in a pass through mode, from either the connected bit line or from a source line connected to the opposite end of the string of floating gate transistors.

As devices continue to reduce dimensions, the ranges of threshold voltage corresponding to individual data values are also becoming smaller. This makes programming more difficult as the differentiation between data values becomes less distinct.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustration of a memory system, according to an embodiment of the present invention.

FIG. 2 illustrates a memory array, according to another embodiment of the invention.

FIG. 3 illustrates waveforms applied to a memory array, according to another embodiment of the invention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.

FIG. 1 is a block diagram illustration of a memory system, according to an embodiment of the present invention. The memory system includes a memory device 100, such as a flash memory device, e.g., NAND flash. Memory device 100 includes a memory array 102 having a plurality of memory cells arranged in row and column fashion. For one embodiment, each of the memory cells may include a floating-gate field-effect transistor capable of holding a charge for the non-volatile storage of data. Each of the cells may be electrically programmed on an individual basis by charging the floating gate.

The rows of memory array 102 may be arranged in blocks, where a memory block is some discrete portion of the memory array 102. Individual word lines generally extend to only one memory block, while bit lines may extend to multiple memory blocks. The memory cells generally can be erased in blocks. Data, however, may be stored in the memory array 102 separate from the block structure.

Memory array 102 can be accessed using externally provided location addresses received by an address register 112 via address signal connections 130. The address signals are decoded, and one or more target memory cells are selected in response to the decoded address signals, using the access circuitry 114 that includes decode and select circuitry.

Data is input and output through an I/O circuit 122 via data connections 132. I/O circuit 122 includes data output registers, output drivers, and output buffers. Command execution logic 124 is provided to control the basic operations of the memory device 100 in response to control signals received via control signal connections 128. For one embodiment, command execution logic 124 includes trim circuitry 125. Trim circuitry 125 is adapted to store control parameter values used by state machine 226 for controlling operations on memory array 102. A state machine 126, that in one embodiment includes high-voltage generation circuitry, may also be provided to control specific operations performed on the memory array and the memory cells. The command execution logic 124 and/or state machine 126 can be generally referred to as control circuitry 127 to control read, write, erase and other memory operations. The control circuitry 127 is adapted to facilitate the methods of the various embodiments. The data connections 132 are typically used for bi-directional data communication. The memory can be coupled to an external processor 150 for operation. An example of a processor 150 includes a memory controller in a personal computer.

Trim circuitry 125 is adapted to store control parameter values used by state machine 126 for controlling operations on memory array 102. Specifically, trim circuitry 125 may include registers that can store the control parameter values after fabrication of the memory device. Exemplary control parameters include parameters for adjusting the magnitude and duration of voltage pulses applied to memory array 102 for carrying out programming and erasing operations.

It will be appreciated by those skilled in the art that additional circuitry and control signals can be provided, and that the memory device of FIG. 1 has been simplified to help focus on the invention. It will further be understood that the above description of a memory device is intended to provide a general understanding of the memory and is not a complete description of all the elements and features of a typical memory device.

FIG. 2 illustrates a NAND memory array 200 as a portion of memory array 102 in accordance with another embodiment of the invention. As shown in FIG. 2, the memory array 200 includes word lines 202 ₁ to 202 _(N) and intersecting local bit lines 204 ₁ to 204 _(M). For ease of addressing in the digital environment, the number of word lines 202 and the number of bit lines 204 are each some power of two, e.g., 256 word lines 202 by 4,096 bit lines 204.

Memory array 200 includes NAND strings 206 ₁ to 206 _(M). Each NAND string includes floating gate transistors 208 ₁ to 208 _(N), each located at an intersection of a word line 202 and a local bit line 204. The floating gate transistors 208 represent non-volatile memory cells for storage of data. The floating gate transistors 208 of each NAND string 206 are connected in series, source to drain, between a source select gate 210, e.g., a field effect transistor (FET), and a drain select gate 212, e.g., an FET. Each source select gate 210 is located at an intersection of a local bit line 204 and a source select line 214, while each drain select gate 212 is located at an intersection of a local bit line 204 and a drain select line 215.

A source of each source select gate 210 is connected to a common source line 216. The drain of each source select gate 210 is connected to the source of the first floating gate transistor 208 of the corresponding NAND string 206. For example, the drain of source select gate 210 ₁ is connected to the source of floating gate transistor 208 ₁ of the corresponding NAND string 206 ₁. A control gate 220 of each source select gate 210 is connected to source select line 214. It is common for a common source line to be connected between source select gates for NAND strings of two different NAND arrays. As such, the two NAND arrays share the common source line.

The drain of each drain select gate 212 is connected to the local bit line 204 for the corresponding NAND string at a drain contact 228. For example, the drain of drain select gate 212 ₁ is connected to the local bit line 204 ₁ for the corresponding NAND string 206 ₁ at drain contact 228 ₁. The source of each drain select gate 212 is connected to the drain of the last floating gate transistor 208 _(N) of the corresponding NAND string 206. For example, the source of drain select gate 212 ₁ is connected to the drain of floating gate transistor 208 _(N) of the corresponding NAND string 206 ₁. It is common for two NAND strings to share the same drain contact.

Typical construction of floating gate transistors 208 includes a source 230 and a drain 232, a floating gate 234, and a control gate 236, as shown in FIG. 2. Floating gate transistors 208 have their control gates 236 coupled to a word line 202. A column of the floating gate transistors 208 is a NAND string 206 coupled to a given local bit line 204. A row of the floating gate transistors 208 are those transistors commonly coupled to a given word line 202.

FIG. 3 illustrates waveforms applied to a memory array, such as memory array 200 of FIG. 2, for programming a target memory cell of a word line that includes the target memory cell. Starting at a time t₁, a voltage SL of source line 216 is increased from a voltage level 302, e.g., about 0 volts, to a voltage level 304, such as Vcc, e.g., about 1.8 volts, at a time t₂. During the time interval between times t₁ and t₂, a voltage SG(D) of drain select line 215 and thus of the control gates of each of drain select gates 212 is increased from a voltage level 306, e.g., about Vss or 0 volts, to a peak voltage level 308, e.g., Vcc, and then is decreased to a voltage level 310 at time t₂. For one embodiment, voltage level 310 is programmed into a register, e.g., of trim circuitry 125 of memory device 100, after fabrication of memory device 100. For another embodiment, the register is a two-bit register that can be programmed to store four different control parameter values corresponding to voltage level 310.

During programming, a voltage SG(S) of source select line 215 and thus of the control gates of each of source select gates 212 is maintained at voltage level 312, e.g., Vss or 0 volts. A voltage BL of bit lines 204 is either maintained at a voltage level 314, e.g., of 0 volts, for programming a logic low or is increased from voltage level 314 at time t₁ to a voltage level 316, such as 1.8 volts, at time t₂. Voltage level 316 corresponds to an inhibit voltage to prevent programming to keep selected memory cells of the bit lines in an erased state and thus leave them unchanged, e.g., at a logic high. For one embodiment, the length the time interval between times t₁ and t₂ is fixed or hard programmed during fabrication of the memory device, e.g., using a metal option.

It will be appreciated by those skilled in the art that the voltage BL can be maintained at voltage level 332 for one or more first bit lines for programming a selected memory cell of each of the one or more first bit lines to a logic low value, and the voltage BL can be increased to voltage level 316 for one or more second bit lines for effectively programming (maintaining) a selected memory cell of each of the one or more second bit lines to a logic high value.

Starting at a time t₂, a voltage WL_(US) of one or more word lines 202 that do not include the target memory cell is increased from a voltage level 320, e.g., about 0 volts, to a voltage level 322 at a time t₃ and remains at voltage level 322 until time t₅. For this situation, voltage level 322 is not sufficient for programming the memory cells and is typically referred to as a pass voltage. This pass voltage has the effect of turning the memory cells of the one or more word lines that do not include the target memory cell to an ON condition regardless of the programmed state of their internal floating gate, allowing them to pass the bit line voltage BL of a bit line to the target memory cell. For one embodiment, voltage level 322 is programmed into a register, e.g., of trim circuitry 125 of memory device 100, after fabrication of memory device 100. For another embodiment, the register is a two-bit register that can be programmed to store four different control parameter values corresponding to voltage level 322.

The voltage WL_(S) of the word line (hereinafter called the selected word line) that includes the target memory cell is increased from voltage level 320 at time t₂ to voltage level 322 at time t₃. The voltage WL_(S) of the selected word line is further increased from voltage level 322 at time t₃ to an initial programming voltage level 324 at a time t₄ and is maintained at voltage level 324 until time t₅. For one embodiment, the initial programming voltage level 324 is programmed into a register, e.g., of trim circuitry 125 of memory device 100, after fabrication of memory device 100. For another embodiment, the register is a two-bit register that can be programmed to store four different control parameter values corresponding to the initial programming voltage level 324.

For one embodiment, the length of the time interval between times t₂ and t₃ is programmed into a register, e.g., of trim circuitry 125 of memory device 100, after fabrication of memory device 100. For another embodiment, the register is a two-bit register that can be programmed to store four different control parameter values corresponding to the length of the time interval between times t₂ and t₃. Note that the length of the time interval between times t₂ and t₃ is the time allowed for the voltage WL_(US) applied to the one or more unselected word lines and the voltage WL_(S) applied to the selected word line to increase from voltage level 320 to voltage level 322, i.e., the pass voltage. For another embodiment, the length of the time interval between times t₃ and t₅ is programmed into a register, e.g., of trim circuitry 125 of memory device 100, after fabrication of memory device 100. For another embodiment, the register is a two-bit register that can be programmed to store four different control parameter values corresponding to the length of the time interval between times t₃ and t₅. Note that the length of the time interval between times t₃ and t₅ is the time that the programming voltage is applied to the selected word line.

At time t₅, voltage WL_(S) is discharged from initial programming voltage level 324 to a voltage level 322, and voltages WL_(S) and WL_(US) are discharged from voltage level 322 to a voltage level 326, e.g., about 0 volts. A program verify is performed, starting after a time t₆. Note that voltages SL and SG(D) are respectively discharged from voltage levels 304 and 310 to voltage levels 328 and 330, e.g., about 0 volts, after voltage WL_(S) and voltage WL_(US) are discharged for one embodiment. For another embodiment, voltage SG(D) is discharged, starting at time t₆, before the program verify, as shown in FIG. 6. Note further that the voltage BL is discharged from voltage level 316 to a voltage level 332, e.g., about 0 volts after voltage WL_(S) and voltage WL_(US) are discharged for one embodiment.

If the program verify indicates that the target memory cell of the selected word line is programmed, the programming is completed. Otherwise, another attempt is made to program the target memory cell of the selected word line by setting the voltages BL, SL, WL_(US), SG(S), and SG(D) as described above and shown in FIG. 3. However, for this programming attempt, a step voltage 334 is added to the initial programming voltage level 324 so that voltage WL_(S) increases, from voltage level 320 at time t₂ to voltage level 322 at time t₃, for one embodiment, and subsequently increases from voltage level 322 at time t₃ to a second programming voltage level 336 at a time t₄, and remains at voltage level 336 until time t₅. Note that the difference between the initial programming voltage level 324 and the second programming voltage level 336 is step voltage 334. At time t₅ and thereafter, the process proceeds as described above and shown in FIG. 3. In particular, voltage WL_(S) is discharged from initial programming voltage level 336 to voltage level 322; voltages WL_(S) and WL_(US) are discharged from voltage level 322 to voltage level 326; and a program verify is performed, starting after time t₆.

If the program verify indicates that the target memory cell of the selected word line is programmed, the programming is completed. Otherwise, another attempt is made to program the target memory cell of the selected word line by setting the voltages BL, SL, WL_(US), SG(S), and SG(D) as described above and shown in FIG. 3 and incrementing voltage WL_(S) from the second programming voltage level 336 by a voltage step 340 to a third programming voltage level 342, as shown in FIG. 3, and the above-described process is repeated to determine whether the target memory cell of the selected word line is programmed. This is repeated in an iterative process until the target memory cell of the selected word line is programmed.

For one embodiment, the voltage steps are programmed in a register, e.g., using trim circuitry 125 of memory device 100, after fabrication of memory device 100. For another embodiment, the register is a two-bit register that can be programmed to store four different control parameter values corresponding to the voltage steps.

Note that when the voltage BL applied to the bit line that is coupled to a NAND string that includes the target memory cell is maintained at the voltage level 314, the difference between the programming voltage and the voltage BL is such that the target memory cell can be programmed when programming voltage reaches an appropriate value. However, when the voltage BL applied to the bit line that is coupled to a NAND string that includes the target memory cell is at the voltage level 316, e.g., an inhibit voltage, the difference between the programming voltage and the voltage BL is such that the target memory cell is prevented from being programmed, e.g., the target memory remains in an erased state.

CONCLUSION

The invention allows for to compensating for variations in processing by allowing a variety of initial values and incremental changes in the programming voltages/times. By permitting variations of initial programming voltages and incremental changes, the performance of the device may be tuned after fabrication. It may even facilitate devices with different performance characteristics, allowing the fabricator to provide variations in programming speed without requiring multiple inventories of devices.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof. 

1. A method of programming a target memory cell of a memory device, comprising: applying a first voltage, at a first time, to a word line that includes the target memory cell; applying a second voltage, at the first time, to word lines that do not include the target memory cell; applying a third voltage, at the first time, to a bit line selectively coupled to a string of memory cells that includes the target memory cell; increasing the first and second voltages at substantially the same rate, starting at a second time after the first time, until the first and second voltages reach a pass voltage level at a third time after the second time; increasing the first voltage from the pass voltage, starting at the third time, to an initial programming voltage level at a fourth time after the third time; maintaining the first voltage at the initial programming voltage level until a fifth time after the fourth time; and determining whether the target memory cell is programmed after the fifth time; and increasing the initial programming voltage level by a step voltage if it is determined that the target memory cell is not programmed; wherein the initial programming voltage, the step voltage, the pass voltage, a length of time between the second and third times, and a length of time between the third and fifth times are each selectable after fabrication of the memory device.
 2. The method of claim 1 further comprises maintaining a fourth voltage applied to a source select line coupled to a control gate of a source select gate that selectively couples a source line to the string of memory cells that includes the target memory cell at Vss from the first time until after the fifth time.
 3. The method of claim 2 further comprises increasing a fifth voltage applied to the source line from about 0 volts, starting at the first time, to Vcc at the second time and maintaining the fifth voltage at Vcc from the second time until after the fifth time.
 4. The method of claim 3 further comprises increasing a sixth voltage applied to a drain select line coupled to a control gate of a drain select gate that selectively couples the bit line to the NAND string that includes the target memory cell from about zero volts, starting at the first time, to Vcc before the second time, decreasing the fifth voltage from Vcc to a third voltage level, at the second time, that is selectable after fabrication of the memory device, and maintaining the sixth voltage at the third voltage level from the second time until after the fifth time.
 5. The method of claim 1 further comprising decreasing the first and second voltages to about 0 volts after the fifth time, but before determining whether the target memory cell is programmed.
 6. The method of claim 1, wherein applying the third voltage, at the first time, to a bit line further comprises maintaining the third voltage at about 0 volts from the first time until after the fifth time or increasing the third voltage from about 0 volts, starting at the first time, to an inhibit voltage at the second time and maintaining the third voltage at the inhibit voltage from the second time until after the fifth time.
 7. A method of programming a memory device, comprising: applying a first voltage, at a first time, to a first word line that includes a plurality of memory cells to be programmed; applying a second voltage, at the first time, to second word lines that include memory cells not to be programmed; applying a third voltage, at the first time, to one or more first bit lines selectively coupled to the first word line and the second word lines for programming one or more first memory cells, of the plurality of memory cells of the first word line to be programmed, at a first logic level; applying a fourth voltage, at the first time, to one or more second bit lines selectively coupled to the first word line and the second word lines for programming one or more second memory cells, of the plurality of memory cells of the first word line to be programmed, at a second logic level; increasing the first and second voltages at substantially the same rate, starting at a second time after the first time, until the first and second voltages reach a pass voltage level at a third time after the second time; increasing the first voltage from the pass voltage, starting at the third time, to an initial programming voltage level at a fourth time after the third time; maintaining the first voltage at the initial programming voltage level until a fifth time after the fourth time; and determining whether the one or more first memory cells are programmed at the first logic level after the fifth time; and increasing the initial programming voltage level by a step voltage if it is determined that the one or more first memory cells are not programmed at the first logic level; wherein the initial programming voltage, the step voltage, the pass voltage, a length of time between the second and third times, and a length of time between the third and fifth times are each selectable after fabrication of the memory device.
 8. The method of claim 7 further comprises maintaining a fifth voltage applied to a source select line, coupled to control gates of source select gates of the memory device, at Vss from the first time until after the fifth time, wherein each source select gate selectively couples a corresponding string of memory cells that includes one of memory cells to be programmed to a common source line.
 9. The method of claim 8 further comprises increasing a sixth voltage applied to the common source line from about 0 volts, starting at the first time, to Vcc at the second time and maintaining the fifth voltage at Vcc from the second time until after the fifth time.
 10. The method of claim 9 further comprises increasing a seventh voltage applied to a drain select line, coupled to control gates of drain select gates of the memory device, from about zero volts, starting at the first time, to Vcc before the second time, decreasing the seventh voltage from Vcc to a third voltage level, at the second time, that is selectable after fabrication of the memory device, and maintaining the seventh voltage at the third voltage level from the second time until after the fifth time, wherein each drain select gate selectively couples a corresponding string of memory cells that includes one of memory cells to be programmed to one of the first or second bit lines.
 11. The method of claim 7 further comprising decreasing the first and second voltages to about 0 volts after the fifth time, but before determining whether the one or more first memory cells are programmed at the first logic level.
 12. The method of claim 7, wherein applying a third voltage, at the first time, to one or more first bit lines further comprises maintaining the third voltage at about 0 volts from the first time until after the fifth time.
 13. The method of claim 12, wherein applying a fourth voltage, at the first time, to one or more second bit lines further comprises increasing the fourth voltage from about 0 volts, starting at the first time, to an inhibit voltage at the second time and maintaining the fourth voltage at the inhibit voltage from the second time until after the fifth time.
 14. A NAND memory device comprising: a memory array comprising: a plurality of rows of memory cells, each row connected to a word line; and a plurality of columns of NAND strings of memory cells, each NAND string selectively connected to a bit line through a drain select gate of the respective column and to a source line through a source select gate of the respective column; control circuitry coupled to the memory array, the control circuitry adapted to perform a method for programming a target memory cell of the memory array, comprising: applying a first voltage, at a first time, to a word line that includes the target memory cell; applying a second voltage, at the first time, to word lines that do not include the target memory cell; increasing the first and second voltages at substantially the same rate, starting at a second time after the first time, until the first and second voltages reach a pass voltage level at a third time after the second time; increasing the first voltage from the pass voltage, starting at the third time, to an initial programming voltage level at a fourth time after the third time; maintaining the first voltage at the initial programming voltage level until a fifth time after the fourth time; determining whether the target memory cell is programmed after the fifth time; and increasing the initial programming voltage level by a step voltage if it is determined that the target memory cell is not programmed; a first register for storing a value corresponding to the pass voltage level, the first register programmable after fabrication and coupled to the control circuitry; a second register for storing a value corresponding to the initial programming level, the second register programmable after fabrication and coupled to the control circuitry; a third register for storing a value corresponding to the voltage step, the third register programmable after fabrication and coupled to the control circuitry; a fourth register for storing a value corresponding to the length of time between the second and third times, the fourth register programmable after fabrication and coupled to the control circuitry; and a fifth register for storing a value corresponding to a length of time between the third and fifth times, the fifth register programmable after fabrication and coupled to the control circuitry.
 15. The NAND memory device of claim 14, wherein the method further comprises maintaining a third voltage applied to a source select line coupled to a control gate of a source select gate that selectively couples a source line to a NAND string of memory cells that includes the target memory cell at Vss from the first time until after the fifth time.
 16. The NAND memory device of claim 15, wherein the method further comprises maintaining a fourth voltage applied to a bit line selectively coupled to the NAND string of memory cells that includes the selected memory cell at about 0 volts from the first time until after the fifth time or increasing the fourth voltage from about 0 volts, starting at the first time, to an inhibit voltage at the second time and maintaining the fourth voltage at the inhibit voltage from the second time until after the fifth time.
 17. The NAND memory device of claim 16, wherein method further comprises increasing a fifth voltage applied to the source line from about 0 volts, starting at the first time, to Vcc at the second time and maintaining the fifth voltage at Vcc from the second time until after the fifth time.
 18. The NAND memory device of claim 17, wherein the method further comprises increasing a sixth voltage applied to a drain select line coupled to a control gate of a drain select gate that selectively couples the bit line to the NAND string that includes the target memory cell from about zero volts, starting at the first time, to Vcc before the second time, decreasing the sixth voltage from Vcc to a third voltage level at the second time, and maintaining the sixth voltage at the third voltage level from the second time until after the fifth time.
 19. The NAND memory device of claim 18 further comprises a sixth register for storing a value corresponding to the third voltage level, the sixth register programmable after fabrication and coupled to the control circuitry.
 20. The NAND memory device of claim 14, wherein the method further comprises decreasing the first and second voltages to about 0 volts after the fifth time, but before determining whether the target memory cell is programmed. 